N2O-based climatology of the Brewer-Dobson Circulation in WACCM, a chemical renalysis and a CTM driven by four dynamical reanalyses

The Brewer-Dobson Circulation (BDC) plays a major role in the stratospheric dynamics in terms of tracer transport through the mean residual meridional advection and the isentropic two-way mixing. The climatological BDC in the Whole Atmosphere Community Climate Model (WACCM) is separated in those components and evaluated through a comparison with a chemical reanalysis of Aura MLS (BRAM2) and with a chemistry-transport model driven by four modern reanalyses (ERA-Interim, JRA-55, MERRA and MERRA2), using the Transformed Eulerian Mean (TEM) analysis of the long-lived tracer N2O and focusing on the vertical residual advection and the horizontal two-way mixing terms.
In the wintertime Southern polar region the horizontal mixing term in WACCM shows near-zero values, while all the reanalyses show strong negative contributions. This disagreement is likely due to the different representation of the polar transport barrier, that affects the mixing inside the polar vortex. In this region the reanalyses are characterized by large uncertainties of the TEM analysis, i.e. the residual term of the budget is quite large (the N2O TEM budget is not fully closed). In the wintertime Northern polar latitudes WACCM shows smaller values of the horizontal mixing term compared to the reanalyses, which show lower uncertainties of the TEM budget. The agreement is improved in the middle and low latitudes, especially in the Northern Hemisphere: the differences are smaller and the residual term is lower compared to the polar latitudes.
The inter-annual variability of the horizontal mixing term is large in the Southern polar latitudes during austral fall and in the Northern polar latitudes during boreal winter.